Optical scanner is a computer peripheral device that employs photoelectric conversion principle to convert scanning object image to digital data. The scanning method mainly includes projecting a linear light source on the scanning object, using an image capturing element to capture the image reflecting from the scanning object, then employing photoelectric conversion principle to convert the object image to digital signals for output.
Referring to FIG. 1A, a conventional optical scanner consists of a document holding board 10 for holding a scanning object 11 thereon which is generally a flat document, and an optical chassis 12 constructed in a modular manner which consists of a linear lamp tube 121, a plurality of reflection mirrors 122, a lens 123 and an image capturing element 124. During scanning operation, the linear lamp tube 121 projects light on the scanning object 11, and the reflection mirrors 122 receive the image reflecting from the scanning object 11. The reflection mirrors 122 are properly positioned such that they can receive the image reflecting from the scanning object 11 and transmit the image of the object 11 to the lens 123 which refracts and forms an image on the image capturing element 124. The image capturing element 124 employs photoelectric conversion principle to convert the image of the object 11 to digital signals. At this time, the scanned image data in the scanner is only a piece of the total object image. Hence a driving means (not shown in the drawing) shall be used to drive the optical chassis 12 to move along the holding board 10 for scanning the object image completely.
However, the scanner usually has to proceed a calibration process before performing the scanning operation. It mainly uses the image capturing element 124 to scan a calibration chart (not shown in the drawing) located inside the upper lid of the scanner. The calibration chart generally has a selected calibration pattern formed in black and white color at a staggered manner according to a specific calibration requirement. The image data obtained after scanning of the calibration chart will be processed to get a calibration parameter. The image capturing element then uses the calibration parameter to perform calibration for setting the left and right hand side margins, scanning starting position value and light intensity compensation and calibration value for the linear lamp tube. As the scanner which has processed the calibration can result in better scanning quality than the one which does not have calibration process, almost all scanners now being used perform calibration process and get the calculation parameter before proceeding formal scanning of the object, and through using the calibration parameter to perform the compensation and calibration for the scanning image to produce a better scanning image quality.
Referring to FIG. 1B, the image scanning process of a scanner adapted a conventional technique includes the steps of: providing a scanning object 131, performing a pre-scanning process and calculating the calibration parameter 132, using the image capturing element of the optical chassis to capture the image of the scanning object 133, performing image calibration and compensation according to the calibration parameter 134, and finishing object scanning 135 and repeating the step 131.
Although the calibration process may enable the scanner to get a better scanning quality for every scanning operation, the calibration process takes a lot of time. In fact, each calibration consumes about 7-8 seconds. Total time for scanning the object generally is slightly over ten seconds. Hence the calibration process takes a relatively large portion of the total scanning time. When the scanner is used for scanning a great amount of document, time being spent and wasted on calibration becomes significant.
Therefore it is desirable to design a scanner that does not need calibration for every scanning operation and which can directly scan the object without affecting scanning quality thereby to save a lot of calibration time and improve utilization efficiency of the scanner.